There has been a need for many years for low cost metal sheet which is characterized by adequate ductility and other desirable physical properties, such that the metal sheet is well adapted to undergo a preliminary forming operation and then a subsequent step of heating to an elevated temperature, such as in brazing, welding, or ceramic and/or resin coating operation. As the heating step occurs at a temperature at which recrystallization of the metal tends to occur rapidly, it is desirable to prevent the coarsening of grain structure during the recrystallization, which would otherwise reduce the strength properties of the article during the process of fabrication. In brazing operations, undue coarsening of the grains is also usually accompanied by the penetration of the molten brazing alloy between the boundaries of the enlarged grains, which can proceed to the extent of perforating the sheet.
Thus, when articles are formed of aluminum alloy sheets having a cladding of aluminum brazing alloy, the assembly of shaped parts is subjected to a temperature above the melting point of the cladding and below the melting point of the core metal. Any substantial coarsening of the metal grains that is brought about through recrystallization of the core alloy at the elevated temperature necessary to effect the brazing then tends to encourage penetration of the molten brazing alloy through the core and cause weakening and ultimate failure of the assembly.
It therefore became highly desirable to provide a clad aluminum brazing sheet so constituted as to prevent or minimize such deleterious grain coarsening on being subjected to a brazing cycle. Generally, it was desirable to provide a procedure for the preparation of aluminum alloy sheet which would impart resistance therein, or remove or minimize an apparently inherent susceptibility thereof, to substantial grain coarsening on recrystallization when subjected to an elevated temperature treatment.
It has therefore been a principal object of this invention to provide novel sheet of wrought aluminum alloy so constituted and structured as to display substantial resistance to grain coarsening at elevated temperatures.
A further object has been to provide a composite aluminum alloy sheet displaying adequate ductility to facilitate its formability together with substantial resistance to grain coarsening when subjected to a brazing operation.
Another objective of the invention has been the provision of a sequence of processing steps adequate to impart to an aluminum alloy sheet physical characteristics enabling ready formability combined with resistance to grain coarsening during a treatment for the bonding thereof at elevated temperatures.
Further objects and advantages of the present invention will appear from the following description.